In a network like a mobile telecommunication network, a primary station (like a base station in GSM or an eNode B in UMTS) indicates the transmission resources allocated to the secondary stations (like mobile stations in GSM or User Equipments in UMTS).
For instance, in current versions of the specifications for the UMTS LTE (Long Term Evolution), signalling is provided on a control channel (like the PDCCH, Physical Downlink Control Channel) transmitted from the primary station (eNB) to the secondary station (UE) to indicate the transmission resources used for data transmission between the eNB and UE.
For downlink data transmissions, each such control signalling message may typically include at least some or all of the following: Time-frequency resource allocation, Number of MIMO layers used (i.e. the number of MIMO transmission beams), Hybrid ARQ process number, Modulation and Coding Scheme (MCS) for each layer, New data indicator (NDI) for each layer, Redundancy version (RV) for each layer, Precoding information for each layer, UE identity, CRC. Usually one control signalling message is transmitted per UE per subframe.
However, for several types of transmission modes, like a Multi User Multiple Input Multiple Output (MU-MIMO) transmission mode for example, where data transmissions to a set of UEs take place in the same (or at least overlapping) time-frequency resources, the amount of overhead arising from the control signalling may therefore become large.